Windows itself will account for a large amount of this. If you reinstalled windows you may have more features running that you had before.
Control panel>add/remove programs>windows components.
if you want to see the larger files you have
Start>search>what size is it>(more than 1mb>search,
when its finished searching click the size column at the top and the results will be sorted into your largest files, in my case usually saved game files(which can be removed).